Electrolyte For Lithium Secondary Battery And Lithium Secondary Battery Including The Same

ABSTRACT

An electrolyte for a lithium secondary battery and a lithium secondary battery including the same are disclosed herein. In some embodiments, an electrolyte for a lithium secondary battery includes a lithium salt, an organic solvent, and a compound represented by Formula 1. In some embodiments, a lithium secondary battery includes a positive electrode, a negative electrode, and the electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Application No. PCT/KR2020/007763, filed on Jun. 16, 2020,which claims priority from Korean Patent Application No.10-2019-0072177, filed on Jun. 18, 2019, the disclosure of which isincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to an electrolyte for a lithium secondarybattery and a lithium secondary battery including the same, and moreparticularly, to an electrolyte for a lithium secondary battery, whichmay suppress a self-discharge phenomenon of the lithium secondarybattery by suppressing dissolution of transition metal in a positiveelectrode, and a lithium secondary battery including the same.

BACKGROUND ART

There is a need to develop technology for efficiently storing andutilizing electrical energy as personal IT devices and computer networksare developed with the development of information society and theaccompanying dependency of society as a whole on the electrical energyis increased.

A technology based on secondary batteries is the most suitabletechnology for various applications, wherein, since a secondary batterymay be miniaturized, it is applicable to a personal IT device, and it isalso applicable to a large device such as a power storage device.

Among these secondary battery technologies, lithium ion batteries, whichare battery systems having the theoretically highest energy density, arein the spotlight.

The lithium ion battery is largely composed of a positive electrodeformed of a transition metal oxide containing lithium, a negativeelectrode capable of storing lithium, an electrolyte that becomes amedium for transferring lithium ions, and a separator, and, among them,a significant amount of research on the electrolyte has been conductedwhile the electrolyte is known as a component that greatly affectsstability and safety of the battery.

The electrolyte causes a reduction decomposition reaction on a negativeelectrode interface during an activation process of the battery to forma solid electrolyte interphase (SEI). The SEI suppresses additionaldecomposition of an electrolyte solution and may transmit lithium ions.

Transition metal ions may be dissolved from a positive electrode activematerial by a decomposition product of a lithium salt included in theelectrolyte under high-temperature conditions, and the dissolvedtransition metal ions may be re-deposited to the positive electrode toincrease resistance of the positive electrode. Also, the dissolvedtransition metal ions may be electrodeposited on the negative electrodeinterface thorough the electrolyte to cause a self-discharge phenomenonof the negative electrode, and may decompose the SEI formed on a surfaceof the negative electrode to reduce passivation ability of the SEI.

Thus, there is an urgent need for research into an electrolyte whichincludes a component capable of scavenging the decomposition product ofthe lithium salt.

Prior Art Document: International Patent Publication No. 2009-157261

DISCLOSURE OF THE INVENTION Technical Problem

An aspect of the present invention provides an electrolyte for a lithiumsecondary battery, which may suppress an electrolyte decompositionreaction and may minimize generation of a lithium salt decompositionproduct and a transition metal ion dissolution phenomenon in a positiveelectrode even when the lithium secondary battery is operated underhigh-temperature conditions, and a lithium secondary battery includingthe same.

Technical Solution

According to an aspect of the present invention, there is provided anelectrolyte for a lithium secondary battery which includes: a lithiumsalt, an organic solvent, and a compound represented by Formula 1.

In Formula 1,

R₁ and R₂ are each independently an alkyl group having 1 to 3 carbonatoms.

According to another aspect of the present invention, there is provideda lithium secondary battery including a positive electrode, a negativeelectrode, and the electrolyte for a lithium secondary battery.

Advantageous Effects

Since an electrolyte for a lithium secondary battery according to thepresent invention may not only suppress an additional electrolytedecomposition reaction and minimize generation of a decompositionproduct of a lithium salt even when the lithium secondary battery isstored or repeatedly charged and discharged at high temperatures but mayalso minimize dissolution of transition metal ions in a positiveelectrode, a lithium secondary battery having improved high-temperaturelife characteristics and resistance characteristics may be prepared.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings attached to the specification illustratepreferred examples of the present invention by example, and serve toenable technical concepts of the present invention to be furtherunderstood together with detailed description of the invention givenbelow, and therefore the present invention should not be interpretedonly with matters in such drawings.

FIG. 1 is a graph illustrating the results of evaluation of resistanceincrease rates (%) after high-temperature (60° C.) storage according toExperimental Example 1;

FIG. 2 is a graph illustrating the results of evaluation of capacityretentions (%) after high-temperature (45° C.) charge and dischargeaccording to Experimental Example 2;

FIG. 3 is a graph illustrating the results of evaluation of resistanceincrease rates (%) after high-temperature (45° C.) charge and dischargeaccording to Experimental Example 3; and

FIG. 4 is a graph illustrating the results of evaluation of resistanceincrease rates (%) and capacity retentions (%) after high-temperature(45° C.) charge and discharge according to Experimental Example 4.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries, and it will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting of thepresent invention. In the specification, the terms of a singular formmay comprise plural forms unless referred to the contrary.

It will be further understood that the terms “include,” “comprise,” or“have” when used in this specification, specify the presence of statedfeatures, numbers, steps, elements, or combinations thereof, but do notpreclude the presence or addition of one or more other features,numbers, steps, elements, or combinations thereof.

Electrolyte for Lithium Secondary Battery

An electrolyte for a lithium secondary battery according to the presentinvention includes: a lithium salt, an organic solvent, and an additiveincluding a compound represented by Formula 1 below.

In Formula 1,

R₁ and R₂ are each independently an alkyl group having 1 to 3 carbonatoms.

(1) Lithium Salt

First, a lithium salt will be described.

The lithium salt is used as a medium for transferring ions in a lithiumsecondary battery, wherein it is desirable that the lithium salt isincluded in a concentration of 0.1 M to 3 M, preferably 0.8 M to 2.5 M,and more preferably 1 M to 1.5 M in the electrolyte for a lithiumsecondary battery. In a case in which the lithium salt is includedwithin the above range, an increase in resistance in the battery may beprevented by preventing decomposition of a solid electrolyte interphase(SEI) formed on an electrode interface when the battery is operated at ahigh voltage while minimizing a by-product generated by the dissolutionof the lithium salt in the electrolyte.

For example, the lithium salt may include at least one compound selectedfrom the group consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄,LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)₂, CF₃SO₃Li, LiC(CF₃SO₂)₃, LiC₄BO₈, LiTFSI,LiFSI, and LiClO₄.

Particularly, among the lithium salts, LiPF₆ is widely used because ithas relatively higher ionic conductivity than other lithium salts.However, in a case in which an organic solvent included in anelectrolyte is decomposed at high temperature, PF₆ ⁻, as an anion, maybe decomposed at high temperature or by-products, such as HF and PF₅,may be generated due to moisture included in the electrolyte. Theby-products, such as HF and PF₅, may be a cause of destructing the SEIon a surface of an electron-rich negative electrode as described aboveor dissolving transition metal ions from a positive electrode.

Thus, in order to suppress a side reaction due to the by-products, thepresent invention aims at providing the compound represented by Formula1, as an additive for forming an SEI, which may scavenge the Lewis acidby-products and may simultaneously suppress the side reaction of theLewis acid by-products at high temperatures by being reduced on thesurface of the negative electrode.

(2) Organic Solvent

Next, the organic solvent will be described.

Various organic solvents typically used in a lithium electrolyte may beused as the organic solvent without limitation. For example, the organicsolvent may include a cyclic carbonate-based organic solvent, a linearcarbonate-based organic solvent, or a mixed organic solvent thereof.

The cyclic carbonate-based organic solvent is an organic solvent whichmay well dissociate the lithium salt in the electrolyte due to highpermittivity as a highly viscous organic solvent, wherein specificexamples of the cyclic carbonate-based organic solvent may be at leastone organic solvent selected from the group consisting of ethylenecarbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate,2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylenecarbonate, and vinylene carbonate, and, among them, the cycliccarbonate-based organic solvent may include at least one of ethylenecarbonate and propylene carbonate (PC).

Also, the linear carbonate-based organic solvent is an organic solventhaving low viscosity and low permittivity, wherein typical examples ofthe linear carbonate-based organic solvent may be at least one organicsolvent selected from the group consisting of dimethyl carbonate (DMC),diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate(EMC), methylpropyl carbonate, and ethylpropyl carbonate, and the linearcarbonate-based organic solvent may specifically include ethyl methylcarbonate (EMC).

It is desirable that an electrolyte having high electrical conductivitymay be prepared if a linear carbonate organic solvent is mixed with acyclic carbonate organic solvent in an appropriate ratio and used as theorganic solvent. Specifically, the cyclic carbonate organic solvent andthe linear carbonate organic solvent may be used by being mixed in avolume ratio of 1:9 to 5:5, for example, 2:8 to 3:7.

Furthermore, the organic solvent may further include an ester (acetatesand propionates) organic solvent having low melting point and highstability at high temperature, for example, a linear ester-based organicsolvent and/or a cyclic ester-based organic solvent in the cycliccarbonate-based organic solvent and/or the linear carbonate-basedorganic solvent to prepare an electrolyte solution having high ionicconductivity.

Typical examples of the linear ester-based organic solvent may be atleast one organic solvent selected from the group consisting of methylacetate, ethyl acetate, propyl acetate, methyl propionate, ethylpropionate, propyl propionate, and butyl propionate.

Also, the cyclic ester-based organic solvent may include at least oneorganic solvent selected from the group consisting of γ-butyrolactone,γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

If necessary, the organic solvent may be further mixed with an amidecompound or a nitrile compound and used.

(3) Additive: Compound Represented By Formula 1

Next, the electrolyte for a lithium secondary battery of the presentinvention includes a compound represented by the following Formula 1 asan additive.

In Formula 1,

R₁ and R₂ are each independently an alkyl group having 1 to 3 carbonatoms.

An electrolyte for a lithium secondary battery generally includes alithium salt such as LiPF₆, and the lithium salt generates a Lewis acidby-product, such as PF₅ or HF, when the lithium salt is decomposed inthe electrolyte under high-temperature conditions. The Lewis acidby-product may dissolve transition metal ions in a positive electrodeactive material, and the dissolved transition metal ions may bere-deposited on a surface of the positive electrode to increaseresistance of the positive electrode. The transition metal ions may beelectrodeposited on a negative electrode interface thorough theelectrolyte and may react with components constituting a solidelectrolyte interphase (SEI) on the electrode interface or an organicsolvent to cause a decomposition reaction, and thus, the transitionmetal ions may reduce passivation ability of the SEI.

Thus, it is necessary to scavenge the Lewis acid compound, as ahigh-temperature decomposition product of the lithium salt, in order toaddress problems, such as an increase in resistance in the battery and adegradation of battery life characteristics, due to the generation ofthe decomposition reaction product by suppressing the decompositionreaction of the SEI under high-temperature conditions.

Thus, in the present invention, the compound represented by Formula 1corresponding to a Lewis base, which may react with the Lewis acid, wasused as the electrolyte additive. Since the compound represented byFormula 1 contains an electron-rich sulfur (S) element in a ring, itacts as the Lewis base. That is, since it provides electrons to theLewis acid decomposition product such as HF or PF₅, it performs a Lewisacid-base reaction with the Lewis acid decomposition product instead ofthe components constituting the SEI formed on the surface of thenegative electrode or a transition metal oxide in the positiveelectrode.

Also, since the compound represented by Formula 1 contains a double bondin its molecular structure, it has excellent reducibility on the surfaceof the negative electrode during initial charge of the lithium secondarybattery. Thus, the compound represented by Formula 1 may preferentiallybe reductively decomposed to improve the components of the SEI so thatthe SEI formed on the surface of the negative electrode may suppress theside reaction of the Lewis acid by-product derived from the lithiumsalt.

Specifically, the compound represented by Formula 1 may include at leastone selected from the group consisting of compounds represented byFormulae 1A to 1C below.

The compound represented by Formula 1 may be included in an amount of0.1 part by weight to 10 parts by weight, preferably 0.1 part by weightto 5 parts by weight, and more preferably 0.1 part by weight to 3 partsby weight based on 100 parts by weight of the electrolyte for a lithiumsecondary battery. In a case in which the compound represented byFormula 1 is included in an amount within the above range, an effectiveSEI may not only be formed on the surface of the negative electrode, butthe Lewis acid by-products derived from the lithium salt, for example,HF and PF₅, may be effectively scavenged. However, in a case in whichthe compound represented by Formula 1 is included in an amount greaterthan the above range, a decomposition reaction may excessively occur toincrease initial resistance of the lithium secondary battery, and, in acase in which the compound represented by Formula 1 is included in anamount less than the above range, an effect as an additive may beinsignificant.

(4) Other Additives

The electrolyte for a lithium secondary battery of the present inventionmay additionally further include other additives which may form a stablefilm on the surfaces of the negative electrode and the positiveelectrode while not significantly increasing the initial resistance inaddition to the effect from the compound represented by Formula 1, orwhich may act as a complementary agent for suppressing the decompositionof the solvent in the electrolyte for a lithium secondary battery andimproving mobility of lithium ions.

These other additives are not particularly limited as long as these areadditives capable of forming a stable film on the surfaces of thepositive electrode and the negative electrode. As a representativeexample, the other additive may include at least one selected from thegroup consisting of a halogen-substituted or unsubstitutedcarbonate-based compound, a vinyl silane-based compound, aphosphate-based compound, a phosphite-based compound, a sulfite-basedcompound, a sulfone-based compound, a sulfate-based compound, asultone-based compound, a halogen-substituted carbonate-based compound,a halogen-substituted benzene-based compound, a nitrile-based compound,a borate-based compound, and a lithium salt-based compound.

Specifically, the other additive may include at least one compoundselected from the group consisting of a vinyl silane-based compound, aphosphate-based compound, a sulfate-based compound, a sultone-basedcompound, a halogen-substituted benzene-based compound, and aborate-based compound.

The halogen-substituted or unsubstituted carbonate-based compound mayinclude vinylene carbonate (VC) or fluoroethylene carbonate (FEC).

The vinyl silane-based compound may improve durability of the battery byforming a stable film through electrochemical reduction on the surfaceof the negative electrode. Specifically, tetravinylsilane (TVS) may beincluded as the vinyl silane-based compound.

The phosphate-based or phosphite-based compound is a component forassisting the formation of the SEI by being electrochemically decomposedon the surfaces of the positive electrode and the negative electrode,wherein an effect of improving long-term cycle life characteristics ofthe secondary battery may be achieved by the phosphate-based orphosphite-based compound. Representative examples thereof may be atleast one compound selected from the group consisting of lithiumdifluoro(bisoxalato)phosphate, lithium difluorophosphate (LiDFP),tris(trimethylsilyl) phosphate (TMSPa), tris(trimethylsilyl) phosphite(TMSPi), tris(2,2,2-trifluoroethyl) phosphate (TFEPa), andtris(trifluoroethyl) phosphite (TFEPi).

The sulfite-based compound may include at least one compound selectedfrom the group consisting of ethylene sulfite, methylethylene sulfite,ethylethylene sulfite, 4,5-dimethylethylene sulfite, 4,5-diethylethylenesulfite, propylene sulfite, 4,5-dimethylpropylene sulfite,4,5-diethylpropylene sulfite, 4,6-dimethylpropylene sulfite,4,6-diethylpropylene sulfite, and 1,3-butylene glycol sulfite.

The sulfone-based compound may include at least one compound selectedfrom the group consisting of divinyl sulfone, dimethyl sulfone, diethylsulfone, methylethyl sulfone, and methylvinyl sulfone.

The sulfate-based compound may include ethylene sulfate (Esa),trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

The sultone-based compound may include at least one compound selectedfrom the group consisting of 1,3-propane sultone (PS), 1,4-butanesultone, ethane sultone, 1,3-propene sultone (PRS), 1,4-butene sultone,and 1-methyl-1,3-propene sultone.

The halogen-substituted benzene-based compound may include fluorobenzene(FB).

Also, the nitrile-based compound may include at least one compoundselected from the group consisting of succinonitrile (SN), adiponitrile(Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile,caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile,difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile,2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

The borate-based compound may include lithium oxalyldifluoroborate(LiODFB), lithium bis(oxalato)borate (LiB(C₂O₄)₂; LiBOB), or lithiumtetrafluoroborate (LiBF₄).

The lithium salt-based compound is a compound different from the lithiumsalt included in the electrolyte, wherein the lithium salt-basedcompound may be LiPO₂F₂.

The compounds listed as the other additives may be included alone or asa mixture of two or more thereof, and may be included in an amount of 1part by weight to 40 parts by weight based on 100 parts by weight of theelectrolyte for a lithium secondary battery, particularly 1 part byweight to 30 parts by weight based on 100 parts by weight of theelectrolyte for a lithium secondary battery, and more particularly 1part by weight to 20 parts by weight based on 100 parts by weight of theelectrolyte for a lithium secondary battery.

If the amount of the other additives is greater than the above range, aside reaction in the electrolyte may occur excessively during charge anddischarge of the battery, and, since an excessive decomposition reactionmay occur at high temperatures, the initial resistance of the lithiumsecondary battery may be increased, or the resistance may becontinuously increased during charge and discharge of the lithiumsecondary battery to degrade discharge capacity and life characteristicsof the battery.

Lithium Secondary Battery

Next, a lithium secondary battery according to the present inventionwill be described.

The lithium secondary battery according to an embodiment of the presentinvention includes a positive electrode, a negative electrode, and theelectrolyte for a lithium secondary battery, and may optionally furtherinclude a separator which may be disposed between the positive electrodeand the negative electrode. In this case, since the electrolyte for alithium secondary battery is the same as described above, a detaileddescription thereof will be omitted.

(1) Positive Electrode

The positive electrode may be prepared by coating a positive electrodecollector with a positive electrode active material slurry including apositive electrode active material, a binder for an electrode, aconductive agent for an electrode, and a solvent.

The positive electrode collector is not particularly limited so long asit has conductivity without causing adverse chemical changes in thebattery, and, for example, stainless steel, aluminum, nickel, titanium,fired carbon, or aluminum or stainless steel that is surface-treatedwith one of carbon, nickel, titanium, silver, or the like may be used.In this case, the positive electrode collector may have fine surfaceroughness to improve bonding strength with the positive electrode activematerial, and the positive electrode collector may be used in variousshapes such as a film, a sheet, a foil, a net, a porous body, a foambody, a non-woven fabric body, and the like.

The positive electrode active material is a compound capable ofreversibly intercalating and deintercalating lithium, wherein thepositive electrode active material may specifically include a lithiumcomposite metal oxide including lithium and at least one metal such ascobalt, manganese, nickel, or aluminum. Specifically, the lithiumcomposite metal oxide may include lithium-manganese-based oxide (e.g.,LiMnO₂, LiMn₂O₄, etc.), lithium-cobalt-based oxide (e.g., LiCoO₂, etc.),lithium-nickel-based oxide (e.g., LiNiO₂, etc.), lithiumiron-phosphate-based positive electrode material (e.g., LiFePO₄),lithium-nickel-manganese-based oxide (e.g., LiNi_(1−Y1)Mn_(Y1)O₂ (whereO<Y1<1), LiMn_(2−Z1)Ni_(z1)O₄ (where O<Z1<2), etc.),lithium-nickel-cobalt-based oxide (e.g., LiNi_(1−Y2)Co_(Y2)O₂ (whereO<Y2<1), lithium-manganese-cobalt-based oxide (e.g.,LiCo_(1−Y3)Mn_(Y3)O₂ (where O<Y3<1), LiMn_(2−Z2)Co_(z2)O₄ (whereO<Z2<2), etc.), lithium-nickel-manganese-cobalt-based oxide (e.g.,Li(Ni_(p1)Co_(q1)Mn_(r1))O₂ (where 0<p1<1, 0<q1<1, 0<r1<1, andp1+q1+r1=1) or Li(Ni_(p2)Co_(q2)Mn_(r2))O₄ (where 0<p2<2, 0<q2<2,0<r2<2, and p2+q2+r2=2), etc.), or lithium-nickel-cobalt-transitionmetal (M) oxide (e.g., Li(Ni_(p3)Co_(q3)Mn_(r3)M_(S1))O₂ (where M isselected from the group consisting of aluminum (Al), iron (Fe), vanadium(V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), andmolybdenum (Mo), and p3, q3, r3, and s1 are atomic fractions of eachindependent elements, wherein 0<p3<1, 0<q3<1, 0<r3<1, 0<S1<1, andp3+q3+r3+S1=1), etc.), and any one thereof or a compound of two or morethereof may be included.

Among these materials, in terms of the improvement of capacitycharacteristics and stability of the battery, the lithium compositemetal oxide may include LiCoO₂, LiMnO₂, LiNiO₂, LiFePO₄, LiMn₂O₄,lithium nickel manganese cobalt oxide (e.g.,Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂, Li(Ni_(0.5)Mn_(0.3)Co_(0.2))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂), or lithium nickel cobalt aluminum oxide(e.g., LiNi_(0.8)CO_(0.15)Al_(0.05)O₂ etc.), and, in consideration of asignificant improvement due to the control of type and content ratio ofelements constituting the lithium composite metal oxide, the lithiumcomposite metal oxide may include Li(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂,Li(Ni_(0.5)Mn_(0.3)Co_(0.2)) Li(Ni_(0.7)Mn_(0.15)Co_(0.15))O₂, orLi(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂, and any one thereof or a mixture of twoor more thereof may be used.

The binder for an electrode is a component that assists in the bindingbetween the positive electrode active material and the electrodeconductive agent and in the binding with the current collector.Specifically, the binder may include polyvinylidene fluoride, polyvinylalcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene,polyethylene (PE), polypropylene, an ethylene-propylene-dieneterpolymer, a styrene-butadiene rubber, styrene-butadienerubber-carboxymethylcellulose (SBR-CMC), a fluoro rubber, variouscopolymers, and the like.

The conductive agent for an electrode is a component for furtherimproving the conductivity of the positive electrode active material.Any conductive agent for an electrode may be used without particularlimitation so long as it has conductivity without causing adversechemical changes in the battery, and, for example, a conductivematerial, such as: graphite; a carbon-based material such as carbonblack, acetylene black, Ketjen black, channel black, furnace black, lampblack, and thermal black; conductive fibers such as carbon fibers ormetal fibers; metal powder such as fluorocarbon powder, aluminum powder,and nickel powder; conductive whiskers such as zinc oxide whiskers andpotassium titanate whiskers; conductive metal oxide such as titaniumoxide; or polyphenylene derivatives, may be used. Specific examples of acommercial conductive agent may include acetylene black-based products(Chevron Chemical Company, Denka black (Denka Singapore PrivateLimited), or Gulf Oil Company), Ketjen black, ethylene carbonate(EC)-based products (Armak Company), Vulcan XC-72 (Cabot Company), andSuper P (Timcal Graphite & Carbon).

The solvent may include an organic solvent, such asN-methyl-2-pyrrolidone (NMP), and may be used in an amount such thatdesirable viscosity is obtained when the positive electrode activematerial as well as optionally the binder for a positive electrode andthe conductive agent for a positive electrode is included.

(2) Negative Electrode

The negative electrode, for example, may be prepared by coating anegative electrode collector with a negative electrode active materialslurry including a negative electrode active material, a binder for anelectrode, a conductive agent for an electrode, and a solvent.

The negative electrode collector is not particularly limited so long asit has high conductivity without causing adverse chemical changes in thebattery, and, for example, copper, stainless steel, aluminum, nickel,titanium, fired carbon, copper or stainless steel that issurface-treated with one of carbon, nickel, titanium, silver, or thelike, an aluminum-cadmium alloy, or the like may be used. Also, similarto the positive electrode collector, the negative electrode collectormay have fine surface roughness to improve bonding strength with thenegative electrode active material, and the negative electrode collectormay be used in various shapes such as a film, a sheet, a foil, a net, aporous body, a foam body, a non-woven fabric body, and the like.

The negative electrode active material may further include at least onecompound selected from the group consisting of a silicon-based compoundrepresented by SiO_(x) (0<x≤2); a graphite-based material such asnatural graphite, artificial graphite, and Kish graphite;lithium-containing titanium composite oxide (LTO); metals (Me) such astin (Sn), lithium (Li), zinc (Zn), Mg, cadmium (Cd), cerium (Ce), nickel(Ni), or Fe; alloys composed of the metals (Me); oxides of the metals(Me); and composites of the metals (Me) and carbon.

Since the binder for an electrode, the conductive agent for anelectrode, and the solvent are the same as described above, detaileddescriptions thereof will be omitted.

(3) Separator

A typical porous polymer film used as a typical separator, for example,a porous polymer film prepared from a polyolefin-based polymer, such asan ethylene homopolymer, a propylene homopolymer, an ethylene-butenecopolymer, an ethylene-hexene copolymer, and an ethylene-methacrylatecopolymer, may be used alone or in a lamination therewith as theseparator, and a polyolefin-based porous polymer film coated withinorganic particles (e.g. Al₂O₃) or a typical porous nonwoven fabric,for example, a nonwoven fabric formed of high melting point glass fibersor polyethylene terephthalate fibers may be used, but the presentinvention is not limited thereto.

Hereinafter, the present invention will be described in detail,according to specific examples. However, the following examples aremerely presented to exemplify the present invention, and the scope ofthe present invention is not limited thereto. It will be apparent tothose skilled in the art that various modifications and alterations arepossible within the scope and technical spirit of the present invention.Such modifications and alterations fall within the scope of claimsincluded herein.

EXAMPLES 1. Example 1 (1) Preparation of Electrolyte for LithiumSecondary Battery

An electrolyte for a lithium secondary battery was prepared by adding0.5 g of the compound represented by Formula 1A, and 0.1 g oftetravinylsilane (hereinafter, referred to as “TVS”), 1 g of lithiumdifluorophosphate (hereinafter, referred to as “LiDFP”), 1 g of ethylenesulfate (hereinafter, referred to as “ESa”), 0.2 g of lithiumtetrafluoroborate (hereinafter, referred to as “LiBF₄”), 6 g offluorobenzene (hereinafter, referred to as “FB”), and 0.5 g of1,3-propane sultone (hereinafter, referred to as “PS”), as otheradditives, to 90.7 g of an organic solvent (ethylene carbonate:ethylmethyl carbonate =3:7 volume ratio) in which 1.0 M LiPF₆ was dissolved.

(2) Lithium Secondary Battery Preparation

A positive electrode active material (Li(Ni_(0.8)Mn_(0.1)Co_(0.1))O₂),carbon black as a conductive agent, and polyvinylidene fluoride (PVDF),as a binder, were mixed in a weight ratio of 97.5:1:1.5 and then addedto N-methyl-2-pyrrolidone (NMP), as a solvent, to prepare a positiveelectrode active material slurry (solid content: 50 wt %). An about 12μm thick aluminum (Al) thin film, as a positive electrode collector, wascoated with the positive electrode active material slurry, dried, andthen roll-pressed to prepare a positive electrode.

A negative electrode active material (SiO:graphite=5:95 weight ratio),carbon black as a conductive agent, and styrene-butadienerubber-carboxymethylcellulose (SBR-CMC), as a binder, were mixed in aweight ratio of 95:1.5:3.5 and then added to water, as a solvent, toprepare a negative electrode active material slurry (solid content: 60wt %). An about 6 μm thick copper (Cu) thin film, as a negativeelectrode collector, was coated with the negative electrode activematerial slurry, dried, and then roll-pressed to prepare a negativeelectrode.

An electrode assembly was prepared by sequentially stacking the positiveelectrode, a polyolefin-based porous separator coated with inorganicparticles (Al₂O₃), and the negative electrode. Thereafter, the electrodeassembly was accommodated in a pouch-type battery case, and theelectrolyte for a lithium secondary battery was injected thereinto toprepare a pouch-type lithium secondary battery.

2. Example 2 (1) Preparation of Electrolyte for Lithium SecondaryBattery

An electrolyte for a lithium secondary battery was prepared by adding0.5 g of the compound represented by Formula 1A, and 0.1 g of TVS, 1 gof LiDFP, 1 g of ESa, 0.2 g of LiBF₄, and 0.5 g of PS, as otheradditives, to 96.7 g of an organic solvent (ethylene carbonate:ethylmethyl carbonate:dimethyl carbonate=3:5:2 volume ratio) in which 1.2 MLiPF₆ was dissolved, when the electrolyte for a lithium secondarybattery was prepared.

(2) Lithium Secondary Battery Preparation

A lithium secondary battery was prepared in the same manner as inExample 1 except that graphite was only used as a negative electrodeactive material.

Comparative Examples 1. Comparative Example 1 (1) Preparation ofElectrolyte for Lithium Secondary Battery

An electrolyte for a lithium secondary battery was prepared by notadding the compound represented by Formula 1A, and adding 0.1 g oftetravinylsilane (hereinafter, referred to as “TVS”), 1 g of lithiumdifluorophosphate (hereinafter, referred to as “LiDFP”), 1 g of ethylenesulfate (hereinafter, referred to as “ESa”), 0.2 g of lithiumtetrafluoroborate (hereinafter, referred to as “LiBF₄”), 6 g offluorobenzene (hereinafter, referred to as “FB”), and 0.5 g of1,3-propane sultone (hereinafter, referred to as “PS”), as otheradditives, to 91.2 g of an organic solvent (ethylene carbonate:ethylmethyl carbonate=3:7 volume ratio) in which 1.0 M LiPF₆ was dissolved.

(2) Lithium Secondary Battery Preparation

A lithium secondary battery was prepared in the same manner as inExample 1 except that the above-prepared electrolyte for a lithiumsecondary battery was used.

2. Comparative Example 2 (1) Preparation of Electrolyte for LithiumSecondary Battery

An electrolyte for a lithium secondary battery was prepared by notadding the compound represented by Formula 1A, and adding 0.1 g of TVS,1 g of LiDFP, 1 g of ESa, 0.2 g of LiBF₄, and 0.5 g of PS, as otheradditives, to 97.2 g of an organic solvent (ethylene carbonate:ethylmethyl carbonate:dimethyl carbonate =3:5:2 volume ratio) in which 1.2 MLiPF₆ was dissolved.

(2) Lithium Secondary Battery Preparation

A lithium secondary battery was prepared in the same manner as inExample 2 except that the above-prepared electrolyte for a lithiumsecondary battery was used.

Experimental Examples 1. Experimental Example 1: Evaluation ofResistance Increase Rate (%) After High-Temperature (60° C.) Storage

After each of the lithium secondary batteries prepared in Example 1 andComparative Example 1 was activated at a constant current (CC) of 0.1 C,degassing was performed.

Thereafter, each lithium secondary battery was charged at a CC of 0.33 Cto 4.20 V under a constant current-constant voltage (CC-CV) condition at25° C., then subjected to 0.05 C current cut-off, and discharged at a CCof 0.33 C to 2.5 V. The above charging and discharging were defined asone cycle, cycles were performed, and, then, after each lithiumsecondary battery was charged to a state of charge (SOC) of 50%, directcurrent internal resistance (hereinafter, referred to as “DC-iR”) wascalculated by a voltage drop obtained in a state in which each lithiumsecondary battery was subjected to a discharge pulse at 2.5 C for 10seconds and was defined as initial resistance. The voltage drop wasmeasured using PNE-0506 charge/discharge equipment (manufacturer: PNESOLUTION Co., Ltd., 5 V, 6 A). Then, each lithium secondary battery wasrecharged at a CC of 0.33 C to a state of charge (SOC) of 100%, thenstored at a high temperature (60° C.) for 2 weeks, charged at a CC of0.33 C to 4.20 V under a constant current-constant voltage (CC-CV)condition, subjected to 0.05 C current cut-off, and discharged at a CCof 0.33 C to 2.5 V.

Next, after each lithium secondary battery was charged to a state ofcharge (SOC) of 50%, DC-iR was calculated by a voltage drop obtained ina state in which each lithium secondary battery was subjected to adischarge pulse at 2.5 C for 10 seconds, and was defined as resistanceafter 2 weeks storage. The voltage drop was measured using PNE-0506charge/discharge equipment (manufacturer: PNE SOLUTION Co., Ltd., 5 V, 6A).

The respectively measured initial resistance and resistance after 2weeks storage were substituted into the following [Equation 1] tocalculate a resistance increase rate (%), and the results thereof arepresented in FIG. 1.

Resistance increase rate(%)={(resistance after 2 weeks storage at hightemperature-initial resistance)/initial resistance}×100  [Equation 1]

Referring to FIG. 1, it may be confirmed that the secondary battery ofExample 1 had a lower resistance increase rate after 2 weeks storage ata high temperature (60° C.) than the secondary battery of ComparativeExample 1.

2. Experimental Example 2: Evaluation of Capacity Retention (%) AfterHigh-Temperature (45° C.) Charge and Discharge

After each of the secondary batteries prepared in Example 1 andComparative Example 1 was activated at a CC of 0.1 C, degassing wasperformed. Thereafter, each secondary battery was charged at a CC of0.33 C to 4.20 V under a constant current-constant voltage (CC-CV)condition at 25° C., then subjected to 0.05 C current cut-off, anddischarged at a CC of 0.33 C to 2.5 V.

Next, each secondary battery was charged at a CC of 0.33 C to 4.20 Vunder a constant current-constant voltage (CC-CV) condition at 45° C.,then subjected to 0.05 C current cut-off, and discharged at a CC of 0.33C to 2.5 V. The above charging and discharging were defined as onecycle, 100 cycles of the charging and discharging were performed at ahigh temperature (45° C.), and discharge capacities were then measuredusing PNE-0506 charge/discharge equipment (manufacturer: PNE SOLUTIONCo., Ltd., 5 V, 6 A). The discharge capacities measured were substitutedinto the following [Equation 2] to calculate capacity retention (%), andthe results thereof are presented in FIG. 2.

Capacity retention(%)=(discharge capacity after 100 cycles/dischargecapacity after 1 cycle)×100  [Equation 2]

Referring to FIG. 2, it may be confirmed that the secondary battery ofExample 1 had a higher battery capacity retention after high-temperaturecharge and discharge than the secondary battery of Comparative Example1.

3. Experimental Example 3: Evaluation of Resistance Increase Rate (%)After High-Temperature (45° C.) Charge and Discharge

After each of the secondary batteries prepared in Example 1 andComparative Example 1 was activated at a constant current (CC) of 0.1 C,degassing was performed.

Thereafter, each secondary battery was charged at a CC of 0.33 C to 4.20V under a constant current-constant voltage (CC-CV) condition at 25° C.,then subjected to 0.05 C current cut-off, and discharged at a CC of 0.33C to 2.5 V. The above charging and discharging were defined as onecycle, 3 cycles were performed, and, then, after each lithium secondarybattery was charged to a state of charge (SOC) of 50%, DC-iR wascalculated by a voltage drop obtained in a state in which each secondarybattery was subjected to a discharge pulse at 2.5 C for 10 seconds, andthe resistance measured was defined as initial resistance. The voltagedrop was measured using PNE-0506 charge/discharge equipment(manufacturer: PNE SOLUTION Co., Ltd., 5 V, 6 A).

Then, 50 cycles, 100 cycles, and 200 cycles of the charging anddischarging were performed under the same charge and dischargeconditions as above at a high temperature (45° C.) In this case, aftereach secondary battery was charged to a state of charge (SOC) of 50% at25° C. after each of the cycles (50 cycles, 100 cycles, and 200 cycles),DC-iR was calculated by a voltage drop obtained in a state in which eachsecondary battery was subjected to a discharge pulse at 2.5 C for 10seconds. A resistance increase rate (%) calculated by substituting theDC-iR into the following [Equation 3] was illustrated in FIG. 3. In thiscase, the voltage drop was measured using PNE-0506 charge/dischargeequipment (manufacturer: PNE SOLUTION Co., Ltd., 5 V, 6 A).

Resistance increase rate(%)={(resistance after n cycles of charging anddischarging−initial resistance)/initial resistance}×100  [Equation 3]

(In Equation 3, n is 50, 100, or 200.)

Referring to FIG. 3, with respect to the secondary battery according toExample 1, since stable films were formed on the surfaces of thepositive electrode and the negative electrode, an additional electrolytedecomposition reaction was suppressed even when charging and dischargingwere performed for a long time at a high temperature (45° C.), and thus,it may be confirmed that the resistance increase rate was lower thanthat of the secondary battery of Comparative Example 1.

4. Experimental Example 4: Evaluation of Resistance Increase Rate (%)and Capacity Retention(%) After High-Temperature (45° C.) Charge andDischarge (1) Resistance Increase Rate (%) Measurement

After each of the secondary batteries prepared in Example 2 andComparative Example 2 was activated at a constant current (CC) of 0.1 C,degassing was performed.

Thereafter, each secondary battery was charged at a CC of 0.33 C to 4.20V under a constant current-constant voltage (CC-CV) condition at 25° C.,then subjected to 0.05 C current cut-off, and discharged at a CC of 0.33C to 2.5 V. The above charging and discharging were defined as onecycle, 3 cycles were performed, and, then, after each secondary batterywas charged to a state of charge (SOC) of 50%, DC-iR was calculated by avoltage drop obtained in a state in which each secondary battery wassubjected to a discharge pulse at 2.5 C for 10 seconds, and theresistance measured was defined as initial resistance. The voltage dropwas measured using PNE-0506 charge/discharge equipment (manufacturer:PNE SOLUTION Co., Ltd., 5 V, 6 A).

Then, after 50 cycles of the charging and discharging were performedunder the same charge and discharge conditions as above at a hightemperature (45° C.), each secondary battery was charged to a state ofcharge (SOC) of 50% at 25° C., and DC-iR was then calculated by avoltage drop obtained in a state in which each secondary battery wassubjected to a discharge pulse at 2.5 C for 10 seconds. A resistanceincrease rate (%) after 50 cycles, which was calculated by substitutingthe DC-iR into the following [Equation 4], was illustrated in FIG. 4(lower graph). In this case, the voltage drop was measured usingPNE-0506 charge/discharge equipment (manufacturer: PNE SOLUTION Co.,Ltd., 5 V, 6 A).

Resistance increase rate(%)={(resistance after 50 cycles−initialresistance)/initial resistance}×100  [Equation 4]

Referring to FIG. 4 (lower graph), with respect to the battery accordingto Example 2, since stable films were formed on the surfaces of thepositive electrode and the negative electrode, an additional electrolytedecomposition reaction was suppressed even when charging and dischargingwere performed for a long time at a high temperature (45° C.), and thus,it may be confirmed that the resistance increase rate was lower thanthat of the battery according to Comparative Example 2.

(2) Capacity Retention (%) Measurement

After each of the secondary batteries prepared in Example 2 andComparative Example 2 was activated at a CC of 0.1 C, degassing wasperformed. Thereafter, each secondary battery was charged at a CC of0.33 C to 4.20 V under a constant current-constant voltage (CC-CV)condition at 25° C., then subjected to 0.05 C current cut-off, anddischarged at a CC of 0.33 C to 2.5 V.

Next, each secondary battery was charged at a CC of 0.33 C to 4.20 Vunder a constant current-constant voltage (CC-CV) condition at 45° C.,then subjected to 0.05 C current cut-off, and discharged at a CC of 0.33C to 2.5 V. The above charging and discharging were defined as onecycle, and, while 50 cycles of the charging and discharging wereperformed at a high temperature (45° C.), discharge capacities weremeasured using PNE-0506 charge/discharge equipment (manufacturer: PNESOLUTION Co., Ltd., 5 V, 6 A). The discharge capacities measured weresubstituted into the following [Equation 4] to calculate capacityretention (%), and the results thereof are presented in FIG. 4(uppergraph).

Capacity retention(%)=(discharge capacity after 50 cycles/dischargecapacity after 1 cycle)×100  [Equation 4]

Referring to FIG. 4 (upper graph), it may be confirmed that thesecondary battery according to Example 2 had a higher capacity retentionthan the secondary battery of Comparative Example 2.

1. An electrolyte for a lithium secondary battery, the electrolytecomprising: a lithium salt; an organic solvent; and a compoundrepresented by Formula 1

wherein, in Formula 1, R₁ and R₂ are each independently an alkyl grouphaving 1 to 3 carbon atoms.
 2. The electrolyte for a lithium secondarybattery of claim 1, wherein the compound represented by Formula 1comprises at least one selected from the group consisting of a compoundrepresented by Formula 1A to a compound represented by Formula 1C


3. The electrolyte for a lithium secondary battery of claim 1, whereinthe compound represented by Formula 1 is present in an amount of 0.1part by weight to 10 parts by weight based on 100 parts by weight of theelectrolyte.
 4. The electrolyte for a lithium secondary battery of claim1, wherein the compound represented by Formula 1 is present in an amountof 0.1 part by weight to 5 parts by weight based on 100 parts by weightof the electrolyte.
 5. The electrolyte for a lithium secondary batteryof claim 1, wherein the compound represented by Formula 1 is present inan amount of 0.1 part by weight to 3 parts by weight based on 100 partsby weight of the electrolyte.
 6. The electrolyte for a lithium secondarybattery of claim 1, wherein the lithium salt comprises LiPF₆.
 7. Theelectrolyte for a lithium secondary battery of claim 1, furthercomprising at least one additive selected from the group consisting of ahalogen-substituted or unsubstituted carbonate-based compound, a vinylsilane-based compound, a phosphate-based compound, a phosphite-basedcompound, a sulfite-based compound, a sulfone-based compound, asulfate-based compound, a sultone-based compound, a halogen-substitutedbenzene-based compound, a nitrile-based compound, a borate-basedcompound, and a lithium salt-based compound.
 8. The electrolyte for alithium secondary battery of claim 7, wherein the other additivecomprises at least one selected from the group consisting of the vinylsilane-based compound, the phosphate-based compound, the sulfate-basedcompound, the sultone-based compound, the halogen-substitutedbenzene-based compound, and the borate-based compound.
 9. Theelectrolyte for a lithium secondary battery of claim 7, wherein theadditive is present in an amount of 1 part by weight to 40 parts byweight based on 100 parts by weight of the electrolyte.
 10. A lithiumsecondary battery, comprising: a positive electrode; a negativeelectrode; and the electrolyte of claim 1.